Electroless gold plating process

ABSTRACT

An Electroless plating bath, and processes for the autocatalytic deposition of Group IB metals upon various substrates using such a bath, are disclosed. The electroless plating bath disclosed includes an aqueous solution of an imide complex of the Group IB metal to be plated, an alkali metal cyanide, and a reducing agent, and is maintained at a pH of from about 11 to 14 by the addition of alkali metal hydroxides. In a preferred embodiment, an electroless gold plating bath is disclosed, including an aqueous solution of an alkali metal gold imide complex. Methods for plating various substrates from such electroless plating baths are also disclosed. Where non-metallic substrates are to be plated, the surfaces are rendered catalytically active prior to immersion in the electroless plating baths, and where metallic substrates are plated, they are preferably pre-coated with a thin plate by immersion in an aqueous plating bath including a soluble gold salt, an ammonium buffering agent, and an organic chelating agent, prior to immersion in the electroless plating baths of the present invention.

[4 1 Nov. 4, 1975 ELECTROLESS GOLD PLATING PROCESS [75] Inventor: Kenneth D. Baker, Somerville, NJ.

[73] Assignee: Engelhard Minerals & Chemicals Corporation, Murray Hill, NJ.

22 Filed: Apr. 26, 1974 21 Appl. No.1 464,666

[52] US. Cl. 427/304; 106/1; 427/305; 427/328; 427/405; 427/437 [51] Int. Cl. C23C 3/02 [58] Field of Search 117/47 A, 130 E; 106/1; 427/304, 305, 328, 405, 437

[56] References Cited UNITED STATES PATENTS 3,230,098 l/1966 Robinson 117/130 E X 3,294,578 12/1966 Popcck 117/130 E X 3,482,974 12/1969 Metley 96/35 3,506,462 4/1970 Oda et a1. 106/1 3,515,571 6/1970 Levy lO6/l X 3,589,916 6/1971 McCormack 117/47 R 3,697,296 10/1972 Bellis 106/1 3,700,469 10/1972 Okinaka 117/130 E OTHER PUBLICATIONS Such et al., Nickle Plating Processes, Chemical Abstracts, Vol. '57, 14863(h), 1962. Bakos et al., Immersion Gold as Sensitizer for Electroless Gold Plating IBM Technical Disclosure Bulletin, September 1972. Science for Electroplaters, Metal Finishing, August 1966.

Primary Examiner-Ralph S. Kendall Assistant ExaminerJohn D. Smith [57] ABSTRACT An Electroless plating bath, and processes for the autocatalytic deposition of Group [B metals upon various substrates using such a bath, are disclosed. The electroless plating bath disclosed includes an aqueous solution of an imide complex of the Group lB metal to be plated, an alkali metal cyanide, and a reducing agent, and is maintained at a pH of from about 1 1 to 14 by the addition of alkali metal hydroxides. In a preferred embodiment, an electroless gold plating bath is disclosed, including an aqueous solution of an alkali metal gold imide complex.

Methods for plating various substrates from such electroless plating baths are also disclosedv Where non-metallic substrates are to be plated, the surfaces are rendered catalytically active prior to immersion in the electroless plating baths, and where metallic substrates are plated, they are preferably pre-coated with a thin plate by immersion in an aqueous plating bath including a soluble gold salt, an ammonium buffering agent, and an organic chelating agent, prior to immersion in the electroless plating baths of the present invention.

20 Claims, No Drawings ELECTROLESS GOLD PLATING PROCESS The present invention relates to improved electroless plating baths for the autocatalytic deposition of Group [B metals upon substrates.

More particularly, the present invention relates to improved electroless silver, copper and gold plating baths for autocatalytic deposition upon catalytic surfaces, which baths provide improved palting characteristics, including improved plating rates.

Still more particularly, the present invention relates to improved methods of plating various substrates utilizing electroless plating baths of the Group IB metal to be plated thereon.

The use of electroless plating baths for the autocatalytic deposition of gold and other Group [B metals upon various substrates has become widely known. Such baths, as compared to conventional electroplating baths, are characterized by an ability to deposit these metals on a wide variety of metallic and nonmetalic substrates without requiring the use of electricity.

For example, in a series of articles published in Plating, beginning in September of 1970, Dr. Okinaka has described the electroless deposition of gold using as reducing agents borohydride and dimethylamine borane, and alkali metal cyanides as the source of gold, with a demonstrable deterioration in the baths with increased cyanide concentrations, particularly upon replenishment with additional alkali metal gold cyanide. In fact, in a November, 1971 article he specifically demonstrates this (see FIG. 2 thereof), and notes that the accumulation of free cyanide ions is known to decrease the deposition (plating) rate. After a single replenishment the author notes some gold precipitation. In addition, Dr. Okinaka discloses that his process results in plating rates of up to about 2.5 microns per hour, and that even as these rates were approached, serious bath instability resulted.

Furthermore, in U.S. Pat. No. 3,589,916, to McCormack, an electroless gold plating bath is described including a water-soluble gold salt and a complexing agent for the gold, in addition to the water-soluble borohydride or amine borane reducing agent, and a stabi lizing amount of a cyanide compound, all maintained at a pH of between 10 and 14. This disclosure thus teaches the use of a complexing agent of form a gold complex in an attempt to prevent gold precipitation during plating. This disclosure also teaches that the amount of water-soluble cyanide compound, between 5 micrograms and 500 milligrams per liter, is critical.

Each of the gold sources previously employed have, however, not been entirely satisfactory. Thus, with the use of the gold cyanide compounds there is a decided deterioration in these plating baths as the cyanide ion concentration increases, particularly upon replenishment therewith. Furthermore, these compounds are not water-soluble to any great extent, and require considerable efforts to maintain in solution, in addition to the fact that the use of these water-soluble gold salts, as described in U.S. Pat. No. 3,589,916, result in the decomposition of the bath.

It is therefore an object of the present invention to provide improved electroless plating baths for the autocatalytic deposition of Group IB metals, including silver, cooper and gold. It is a further-object of the present invention to provide such electroless plating baths having improved plating rates, and plating lives. It is 2 still another object of the present invention to provide such electroless plating baths which result in the deposition of Group IB metal plates of improved characteristics, including better adhesion, thickness, brightness and ductility.

It is yet another objectof the present invention to provide methods for plating .various metallic and nonmetallic substrates utilizing such electroless plating baths.

In accordance with the present invention electroless plating baths for autocatalytic deposition upon various substrates are provided, resulting in the deposition of thick silver, copper and gold plates. The electroless plating baths of this invention comprise an aqueous so lution of an imide complex of the Group IB metal to be plated, an alkali metal cyanide in an amount sufficient to stabilize the bath, and a reducing agent selected from the group consisting of water soluble alkali metal borohydrides, water soluble amine boranes and formaldehyde. The pH of the bath is maintained from about 1 l to 14. It has thus been found that these electroless plating baths exhibit superior plating rates, without concomitant bath decomposition. Specifically, plating rates of greater than 2.5 microns per hour, and preferably greater than 2.7 microns per hour are obtainable therewith.

In a preferred embodiment, the required pH level is maintained by the addition of alkali metal hydroxides to the bath, and preferably alkali metal buffering salts are added thereto in addition to the alkali metal hydroxides, in order to obtain a superior degree of pH control.

In addition, certain organic chelating agents may be added to these electroless plating baths, particularly when metallic substrates are to be plated therewith, in order to form complexes with the replaced metals, and prevent the precipitation thereof.

Further in accordance with the present invention, improved methods for plating various substrates are provided, in which the metallic or non-metallic substrates are rendered catalytically active prior to immersion to the improved electroless plating baths hereof. Ductile silver, copper and gold plates are thus provided, having strong adherence to the substrates employed, and of relatively uniform thicknesses as compared to the plates obtainable by the prior art plating baths.

It has therefore been found that highly improved electroless plating baths are obtained when an imide complex of the Group IB metal to be plated is employed as the source of that metal, such as gold. Such imide compounds thus form unexpectedly strong complexes with the Group [B metals to be plated, therefore preventing precipitation of the Group 18 metal during plating, while at the same time being fully soluble in the plating solution, and being sufficiently stable so as not to react with the reducing agent prior to deposition.

The imides of the present invention, which are capable of forming particularly strong complexes with the Group ID metals to be plated, such as gold, have a general formula as follows:

wherein R is a radical selected from the group consisting of alkylene, substituted alkylene, arylene, and substituted arylene. Preferably, R will be a substituted arylene, such as sulfonyl-o-phenylene (SO C H in which case the imide formed will be sulphobenzoic imide, (i.e., saccharin, or o-benzosulfimide) C H (S0 (C .))--Nl-l. Thus, for the preparation of an electroless gold plating bath, complexes of thses imides with gold are prepared, such as alkali metal gold sulphobenzoic imide.

Preferably, the imides with which the Group 18 metals to be plated are complexed will have a general formula as follows:

RCONHCO,

wherein R is a radical selected from the group consisting of alkylene, substituted alkylene, arylene, and substituted arylene.

Preferably, R will be alkylene. For example, where R is C H the imide will be succinimide, and where R is an arylene radical such as C l-i (o-phenylene) the imide will be phthalimide. Where the preparation of an electroless gold-plating bath is desired, these two compounds, succinimide and phthalimide, will be particularly preferred, thus forming complexes with the gold in the form of alkali metal gold succinimide, particularly potassium gold succinimide, and alkali metal gold phthalimide, particularly potassium gold phthalimide.

It is also essential that the electroless plating baths of this invention include soluble cyanide compounds in critical amounts in order to maintain the stability of the bath.

Among these water soluble cyanide compounds are the alkali metal cyanides, such as sodium, potassium and lithium cyanide. Among these, sodium and potassium cyanide are particularly preferred. Other such compounds may, however, be employed, such as nitriles, including alpha-hydroxynitriles, etc. as described in U.S. Pat. No. 3,589,916 which compounds are thus incorporated herein by reference. It is critical, however, that the water-soluble cyanide compounds be used in specific amounts, generally from between about 2 and 20 grams per liter, and preferably from about 5 to grams per liter. It has thus been discovered that at below about 2 grams per liter the electroless plating baths of this invention are relatively unstable, and the metal to be plated precipitates from the bath, and that when the cyanide compound concentration increases to about grams per liter, the plating rate obtainable with these baths rapidly deteriorates. Thus, the use of those amounts of water soluble cyanide compounds described in U.S. Pat. No. 3,589,916, which as described above are considerably below those taught herein, results in the preparation of highly unstable electroless plating baths.

The reducing agents employed in connection with the present electroless plating baths include any of the borohydrides or amine boranes which are soluble and stable in aqueous solution. Thus, alkali metal borohydrides, preferably sodium and potassium borohydrides, are utilized, although various substituted borohydrodes, such as sodium or potassium trimethoxyborohydride, Na[K]B(OCH I-l, may also be employed. Also preferred are the amine boranes such as monoand dilower alkyl, e.g., up to C alkyl-amine boranes, preferably isopropyl amine borane and dimethylamine borane. In addition, formaldehyde is also an excellent reducing agent for use in these baths, particularly for the deposition of copper and silver.

It is also essential that the electroless plating baths of the present invention be maintained at a pH of between about 11 and 14, principally to prevent spontaneous decomposition thereof. It is thus preferred that an alkali metal hydroxide, such as sodium or potassium hydroxide, be employed to maintain the pH at this level. It has, however, been discovered that pH control is considerably easier when alkali metal buffering salts are employed in addition to the alkali metal hydroxide. Thus, while the alkali metal hydroxide is necessary in order to maintain the present electroless baths at the required pH level during plating, as the pH tends to drop, the ease of pH control is considerably facilitated by the addition of such alkali metal buffering salts. These alkali metal buffering salts thus include the alkali metal phosphates, citrates, tartrates, borates, metaborates, etc. Specifically, the alkali metal buffering salts may this include sodium or potassium phosphate potassium pyrophosphate, sodium or potassium citrate, sodium potassium tartrate, sodium or potassium borate, sodium or potassium metaborate, etc. The preferred alkali metal buffering salts are sodium or potassium citrate and sodium or potassium tartrate. in order to further improve the electroless plating baths of this invention, it is preferred to add an organic chelating agent thereto. Such chelating agents combine with the replaced surface metal ions, thus preventing their interference with the plating process, and the consequent deterioration of the color characteristics of the deposited plates, as well as other plate properties such as adherence and thickness. These organic chelating agents thus include ethylenediamine tetraacetic acid, and the di-sodium, tri-sodium and tetra-sodium and potassium salts of ethylenediamine tetraacetic acid, di-ethylene triamine pentacetic acid, nitrilotriacetic acid. The ethylenediamine tetraacetic acid and its di-, tri-, and tetrasodium salts are the preferred chelating agents, with the triand tetra-sodium salts being particularly preferred.

In order to autocatalytically deposit Group 18 metal plates, such as gold plates, in accordance with this invention, it is necessary to contact these electroless plating solutions with a catalytically active substrate surface. Thus, where a metallic substrate is employed, such surfaces include all metals which are catalytic to the reduction of the metal cations dissolved in the described baths. While it therefore may in some cases be preferred to further sentitize the substrate by treatments well known to those skilled in this art, the use of nickel, cobalt, iron, steel, paladium, platinum, copper, brass, manganese, chromium, molybdenum, tungsten, titanium, tin, silver, etc., as metal substrates upon which the Group 18 metal, such as gold, is to be plated, are possible.

With the use of non-metallic substrates, however, these surfaces must be rendered catalytically active by producing a film of particles of catalytic material thereon. This may be done in the method described in U.S. Pat. No. 3,589,916, upon such surfaces as glass, ceramics, various plastics. etc. Preferably, when a plastic substrate is to be plated according to the present invention, it is initially etched, preferably in a solution of chromic and sulfuric acid. After rinsing, the substrate is immersed in an acidic solution of stannous chloride, such as stannous chloride and hydrochloric acid, rinsed with water and then contacted with an acid solution of a precious metal, such as paladium chloride in hydrochloric acid. Subsequently, the now catalytically acitve non-metallic substrate may be contacted with the electroless plating solutions of this invention in order to autocatalytically deposite Group 18 metal plates thereon.

In a highly preferred embodiment of the present invention, where a metallic substrate is to be plated a preplating step is employed. Thus, it has been discovered that when the catalytically active metallic substrates of this invention are employed directly with the electroless plating baths hereof a rapid contamination of the plating bath with the base metals replaced by the Group lB metals plated thereon occurs, thus affecting the purity of the deposit obtained, and the stability of the electroless plating solution. It is therefore preferred that, in such situations, a pre-plating step for the provision of a thin plate of gold be carried out. This may be accomplished by using the Atomex process as described in US. Pat. No. 3,230,098, wherein the metallic substrate is immersed in an aqueous gold-plating bath including a soluble gold salt selected from the group consisting of the alkali metal gold cyanides, an ammonium buffering agent capable of maintaining the pH of the bath from between about 5.5 and 14, and an organic chelating agent capable of chelating with the metal ions of the substrate employed.

After a thin film, i.e. from between about 2 to micro-inches, preferably 5 to 10 microinches of gold, is plated by such a method, the plated substrate may be immersed in the electroless plating baths of the present invention in order to provide a thick, ductile plate of the Group 18 metal having the improved properties obtainable in accordance with the present invention.

Typical plating baths made in accordance with the present invention follow:

EXAMPLE 1 A gold solution is obtained by dissolving potassium gold succinimade in water, and heating to between about 70 and 95C. Potassium cyanide, tripotassium citrate, potassium hydroxide, and ethylene diamine tetraacetic acid are added thereto, the pH being maintained at about 13 and the temperature at about 80C. Dimethylamine borane is then dissolved in water, and added to this solution. In order to continue the gold plating, gold is replenished from an aqueous solution containing gold, as potassium gold succinimide. Additional amounts of dimethylamine borane may be required, as well as potassium hydroxide in order to maintain the proper pH.

The initial plating bath thus prepared has the following composition:

Gold, as potassium gold succinimide 3.0 grams per liter Potassium cyanide 5.0 grams per liter EDTA (tetra sodium salt) 2.0 grams per liter Tripotassium citrate 25.0 grams per liter Potassium hydroxide 10.0 grams per liter Dimethylamine borane 10.0 grams per liter EXAMPLE 2 A silver solution is obtained by dissolving the sodium silver phthalirnide in water, along with sodium potassium tartrate, potassium silver succinimide, and potassium hydroxide. The solution is then heated to approximately 55C, and a 37% formaldehyde solution is added thereto.

The silver plating bath thus prepared has the following composition:

Silver, as sodium silver phthalimide 2.0 grams per liter Potassium cyanide 10 grams per liter Sodium potassium tartrate 25 grams per liter Potassium hydroxide 5 grams per liter Formaldehyde (37%) 50 milliliters per liter EXAMPLE 3 A copper solution is obtained by dissolving potassium copper sulphobenzoic imide in an aqueous solution of sodium potassium tartrate, potassium cyanide. and potassium hydroxide. This solution is then heated to approximately 30C, and a 37% solution of formaldehyde is added thereto.

The copper bath thus prepared has the following composition:

Copper, as potassium copper sulphobenzoic imide 3 grams per liter Potassium cyanide 2 grams per liter Sodium potassium tartrate 30 grams per liter Potassium hydroxide 10 grams per liter Formaldehyde (37%) 2O milliliters per liter.

What is claimed is:

1. An electroless plating bath for autocatalytic deposition of Group IB metals upon a substrate comprising an aqueous solution of an imide complex of the Group 13 metal to be plated, said imide complex of the Group IB metal to be plated including an imide selected from the group consisting of imides having the formula H O, and cyclic imides having the formula R NH 0, wherein R is selected from the group consisting of alkylene, substituted alkylene, arylene, and substituted arylene, an alkali metal cyanide in an amount ranging from about 2 to 20 grams per liter, suf ficient to stabilize said bath, and a reducing agent selected from the group consisting of water soluble alkali metal borohydrides, water soluble amine boranes and formaldehyde, said bath maintained at a pH of from about 11 to 14.

2. The electroless plating bath of claim 1, wherein said Group [B metal to be plated is selected from the group consisting of gold, silver and copper.

3. The electroless plating bath of claim 1, wherein said bath is maintained at a pH of from about ll to 14 by the addition of alkali metal hydroxide thereto.

4. The electroless plating bath of claim 3, including an alkali metal buffering salt selected from the group consisting of alkali metal phosphates, citrates, tartrates, borates, metaborates, and mixtures thereof.

5. The electroless plating bath of claim 1, wherein said imide is selected from the group consisting of succinimide and phthalimide.

6. An electroless gold plating bath for the autocatalytic deposition of gold upon a substrate, comprising an aqueous solution of an alkali metal gold imide complex, said alkali metal gold imide complex including an imide selected from the group consisting of imides having the formula RNHQO, and cyclic imides having the formula lECONHCO, wherein R is selected from the group consisting of alkylene, substituted alkylene, arylene, and substituted arylene, an alkali metal cyanide in an amount sufficient to stabilize said bath, a reducing agent selected from the group consisting of water soluble alkali metal borohydrides, water soluble amine boranes, and formaldehyde, said bath maintained at a pH of from about 11 to 14.

7. The electroless plating bath of claim 6, wherein said imide comprises sulfobenzoicimide.

8. The electroless gold plating bath of claim 6, wherein said cyclic imide is selected from the group consisting of succinimide and phthalimide.

9. The electroless gold plating bath of claim 6 including an organic chelating agent capable of forming a chelate with the metal of said substrate.

10. The electroless gold plating bath of claim 6, wherein said bath is maintained at a pH of from about 11 to 14 by the addition of an alkali metal hydroxide thereto.

11. The electroless gold platng bath of claim 10, including an alkali metal buffering salt selected from the group consisting of an alkali metal phosphates, citrates, tartrates, borates, metaborates, and mixtures thereof.

12. The electroless gold plating bath of claim 6, wherein said reducing agent is selected from the group consisting of dimethylamine borane, potassium borohydride, and formaldehyde.

13. A method of plating a metallic substrate with a Group [B metal comprising:

(a) immersing said substrate in an aqueous gold plating bath comprising a soluble gold salt selected from the group consisting of the alkali metal gold cyanides, an ammonium buffering agent capable of maintaining the pH of said bath between about 5.5 and 14, an organic chelating agent capable of chelating the metal ions of said substrate, for sufficient time to place said substrate with a thin layer of gold; and

(b) subsequently immersing said partially gold plated metal substrate in an electroless plating bath comprising an aqueous solution of a Group [8 metal imide complex, said Group lB metal imide complex including an imide selected from the group consisting of the imides having the formula RNHCO, and the cyclic imides having the formula RNHCO- wherein R is selected from the group consisting of alkylene, substituted alkylene, arylene, and substituted arylene, and substitute therefor alkali metal cyanide in an amount of from about 2 to 20 grams/liter, sufficient to stabilize said bath, and a reducing agent selected from the group consisting of water soluble alkali metal borohydrides, water soluble amine boranes and formaldehyde, said bath maintained at a pH of form about 11 to 14.

14. The method of claim 13, wherein said layer of gold plated in step (a) is from about 2 to 10 microinches. v

15. The method of claim 13, wherein said electroless plating bath is maintained at a pH of from about 1 l to 14 by the addition of alkali metal hydroxide thereto.

16. The method of claim 15, wherein said electroless plating bath includes an alkali metal buffering salt.

17. The method of claim 13, wherein said electroless plating bath includes an organic chelating agent capable of forming a chelate with the metal of said substrate.

18. The method of claim 1, wherein said imide comprises sulfobenzoicimide.

19. The method of claim 13, wherein said imide comprises an imide selected from the group consisting of succinimide and phthalimide.

20. The metal of claim 13, wherein said Group 18 metal comprises gold.

UNITED sTATEs PATENT AND TRADEMARK OFFICE CERTIFEfiATE OF CGRREQTEGN PATENTNO. 3,9'17, DATED November 4, 1975 lNV ENTOR(S) Kenneth D. Baker lt'is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: C

In column 2, line 62, "ANCHO" should be RNHCO In column 7, line 31 "JENHC?" should be -RCONHCO- In column 8, line 3, after "arylene" and before "elkali" delete the phrase, [and substitute therefor] In Column 8, at the end of line 8, delete "form" and substitute from-- Signed. and Sealed this [SEAL] twenty-ninth Day or June 1976 Attest:

t RUTH C. M ASO Arresting Officer C) MARSHALL DANN i l mmlssmner 0] Patents and Trademarks [a l 

1. AN ELECTROLESS PLATING BATH FOR AUTOCATALYTIC DEPOSITION OF GROUP 1B METALS UPON A SUBSTRATE COMPRISING AN AQUEOUS SOLUTION OF AN IMIDE COMPLEX OF THE GROUP 1B METAL TO BE PLATED, SAID IMIDE COMPLEX OF THE GROUP 1B META TO BE PLATED INCLUDING AN IMIDE SELECTED FROM THE GROUP CONSISTING OF IMIDES HAVING THE FORMULA RNHCO, AND CYCLIC IMIDES HAVING THE FORMULA RCONHCO, WHEREIN R IS SELECTED FROM THE GROUP CONSISTING OF ALKYLENE, SUBSTITUTED ALKYLENE, ARYLENE, AND SUBSTITUTED ARYLENE, AN ALKALI METAL CYANIDE IN AN AMOUNT RAGING FROM ABOUT 2 TO 20 GRAMS PER LITER, SUFFICIENT TO STABILIZE SAID BATH, AND A REDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF WATER SOLUBLE ALKALI METAL BOROHYDRIDES, WATER SOLUBLE AMINE BORANES AND FORMALDEHYDE, SAID BATH MAINTANED AT A PH OF FROM ABOUT 11 TO
 14. 2. The electroless plating bath of claim 1, wherein said Group IB metal to be plated is selected from the group consisting of gold, silver and copper.
 3. The electroless plating bath of claim 1, wherein said bath is maintained at a pH of from about 11 to 14 by the addition of alkali metal hydroxide thereto.
 4. The electroless plating bath of claim 3, including an alkali metal buffering salt selected from the group consisting of alkali metal phosphates, citrates, tartrates, borates, metaborates, and mixtures thereof.
 5. The electroless plating bath of claim 1, wherein said imide is selected from the group consisting of succinimide and phthalimide.
 6. An electroless gold plating bath for the autocatalytic deposition of gold upon a substrate, comprising an aqueous solution of an alkali metal gold imide complex, said alkali metal gold imide complex including an imide selected from the group consisting of imides having the formula RNHCO, and cyclic imides having the formula RCONHCO, wherein R is selected from the group consisting of alkylene, substituted alkylene, arylene, and substituted arylene, an alkali metal cyanide in an amount sufficient to stabilize said bath, a reducing agent selected from the group consisting of water soluble alkali metal borohydrides, water soluble amine boranes, and formaldehyde, said bath maintained at a pH of from about 11 to
 14. 7. The electroless plating bath of claim 6, wherein said imide comprises sulfobenzoicimide.
 8. The electroless gold plating bath of claim 6, wherein said cyclic imide is selected from the group consisting of succinimide and phthalimide.
 9. The electroless gold plating bath of claim 6 including an organic chelating agent capable of forming a chelate with the metal of said substrate.
 10. The electroless gold plating bath of claim 6, wherein said bath is maintained at a pH of from about 11 to 14 by the addition of an alkali metal hydroxide thereto.
 11. The electroless gold platng bath of claim 10, including an alkali metal buffering salt selected from the group consisting of an alkali metal phosphates, citrates, tartrates, borates, metaborates, and mixtures thereof.
 12. The electroless gold plating bath of claim 6, wherein said reducing agent is selected from the group consisting of dimethylamine borane, potassium borohydride, and formaldehyde.
 13. A method of plating a metallic substrate with a Group IB metal comprising:
 14. The method of claim 13, wherein said layer of gold plated in step (a) is from about 2 to 10 microinches.
 15. The method of claim 13, wherein said electroless plating bath is maintained at a pH of from about 11 to 14 by the addition of alkali metal hydroxide thereto.
 16. The method of claim 15, wherein said electroless plating bath includes an alkali metal buffering salt.
 17. The method of claim 13, wherein said electroless plating bath includes an organic chelating agent capable of forming a chelate with the metal of said substrate.
 18. The method of claim 1, wherein said imide comprises sulfobenzoicimide.
 19. The method of claim 13, wherein said imide comprises an imide selected from the group consisting of succinimide and phthalimide.
 20. The metal of claim 13, wherein said Group IB metal comprises gold. 